Accurate effective fluid approximation for ultralight axions

TL;DR

This paper introduces a novel effective fluid method to accurately and efficiently model ultralight axions in cosmology, enabling precise predictions of their impact on structure formation without computationally expensive oscillation tracking.

Contribution

The authors develop an exact effective fluid representation for ultralight axions and approximate it with an equation of state, achieving subpercent accuracy in linear theory predictions.

Findings

Achieved subpercent accuracy in axion density fluctuation suppression.

Provided a fitting formula for the matter power spectrum transfer function.

Simplified implementation in cosmological codes for future experiments.

Abstract

Ultralight axions are theoretically interesting and phenomenologically rich dark sector candidates, but they are difficult to track across cosmological timescales because of their fast oscillations. We resolve this problem by developing a novel method to evolve them efficiently and accurately. We first construct an exact effective fluid which at late times matches the axion but which evolves in a simple way. We then approximate this evolution with a carefully chosen equation of state and sound speed. With our scheme we find that we can obtain subpercent accuracy for the linear theory suppression of axion density fluctuations relative to that of cold dark matter without tracking even a single complete oscillation of the axion field. We use our technique to test other approximation schemes and to provide a fitting formula for the transfer function for the matter power spectrum in linear theory in axion models. Implementing our approach in existing cosmological axion codes is straightforward and will help unleash the potential of high-precision next-generation experiments.